Open loop driven stepper motors are commonly used to power the actuators of low cost disk drives. They are relatively inexpensive, and they offer sufficient positioning accuracy and repeatability to satisfy the requirements of most floppy disk drives and even some Winchester disk drives. For example, such stepper motors have been successfully employed in disk drives to power band drive actuators of the type described in U.S. Pat. No. 4,161,004, which issued July 10, 1979 on a "Head Positioning Mechanism for Recording/Playback Machine." More particularly, the Shugart SA850 disk drive is an example of a floppy disk drive which has an open loop stepper motor for operating the linear band drive actuator described in that patent, while the Shugart SA4000 is an example of a Winchester disk drive which has an open loop stepper motor for operating the rotary band drive actuator described in that patent.
It is well understood that stepper motors inherently are substantially under damped systems. Stopping torque is developed internally of such a motor only after the rotor of the motor has rotated beyond its stable detent or step position. Thus, the rotor tends to overshoot and oscillate about the desired step position at a so-called "natural frequency", thereby prolonging the motor settling time. Stepper motors also have a tendency to exhibit unwanted "mid-frequency" resonance which retards their acceleration.
Effective damping is the key to controlling the natural resonance and the mid-frequency resonance of a stepper motor. Accordingly, substantial effect and expense have been devoted to the development of open and closed loop damping systems for such motors. Closed loop damping systems are beyond the scope of the present invention, but it is noted that some designers have turned to those more costly systems for damping the actuators of so-called low cost disk drives in an attempt to meet the challenge of the shortened seek/settle times which are being specified for modern disk drives. As a matter of definition, "seek time" is used hereinbelow to refer to (1) the time that is required for the read/write head of a disk drive to move to a selected data track, plus (2) the time that is required for the head to settle on that track. However, others might prefer to call the sum of those two time periods the seek/settle time as hereinabove.
For most disk drive applications, single track seek time is a critically important factor only if it exceeds the so-called latency time (i.e., the time required for the disk to make a complete revolution). If the single track seek time is shorter than the latency time, the latency period determines the average single track data access time of the disk drive, which is the parameter that is usually of interest to the user.
As will be appreciated, single track seek time is the shortest relevant seek time of a disk drive. However, even the seemingly unambitious goal of ensuring that the single track seek time is less than the latency period has been difficult to achieve while using an open loop stepper motor for the actuator, even though proposals have been made to reduce the step rate as the actuator completes its seek and to mechanically damp the stepper motor, such as with a viscous damper as in the Shugart SA1000 disk drive or with a frictional retard mechanism as in the aforementioned patent. Viscous dampers have been favored for mechanical damping because of their relatively straightforward first order damping characteristics, but they are relatively complex to manufacture and are a potential source of unwanted contaminants, especially in a Winchester disk drive environment where cleanliness is critical. For a more comprehensive discussion of the prior art, see Kordik, Kenneth S., "Reducing Overshoot and Oscillation in Variable Reluctance Step Motors," Proceedings, Ninth Annual Symposium on Incremental Motion Control Systems and Devices, University of Illinois, June 2-5, 1980, pp. 65-78.